In variable reluctance (VR) stepper motors, plural stator windings are series connected in separate groups and associated control circuitry successively energizes the winding groups to impart incremental motion to the rotor shaft of the motor. Upon each successive such winding group energization, the rotor rotates until a pole thereof reaches a position of minimum magnetic reluctance relative to a stator pole. The number of such minimum magnetic reluctance positions is related to the numbers of stator and rotor poles and determines the rotor incremental angular rotation occurring on each stator energization. Where VR motors are intended for general usage and particularly at stepping rates inclusive of the resonant stepping rate, damper arrangements are desirably incorporated in the motor to minimize the overshooting of minimum magnetic reluctance positions and thus avoid rotor hunting. Various such damper arrangements are presently known, for example, viscous coupled inertia dampers and electromagnetic dampers exerting continuous drag on the rotor during rotation thereof.
While effective for their intended purpose, such known damper arrangements do not serve to maintain the rotor of a VR motor in fixed position upon motor deenergization. Thus, there is no capability in such known damper arrangements for guarding against rotor drift, i.e., random rotational movements due to environmental disturbances. As respects the viscous type damper, rotor drift can be in the form of rotational movement below the threshold sensitivity level thereof. As for the electromagnetic type dampers, the same are operative only when the stator is energized. Furthermore, known damper arrangements are not rotational position selective in exerting restraining force on VR motor rotors.
The shortcomings of existing VR stepper motors in respect of drift prevention or minimization take on added significance in application thereof to high accuracy linear output control. Thus, in applications within the immediate purview of the present invention, such as those wherein rotor motion is translated by a lead screw into rectilinear displacement of an event-detecting element, rotor drift and hence lead screw movement are matters of critical concern and existing VR motor arrangements are not particularly well suited for use.